In fast neutron nuclear power stations, heat exchangers are used in which the exchanger fluids are constituted by liquid sodium which constitutes the primary and secondary coolant fluid of the reactor, and by water which, by evaporation, supplies the fluid driving the turbine.
It is extremely important to monitor the integrity of the exchange surfaces separating the fluids in these exchangers, in order to detect the presence or appearance of small fissures in these exchange surfaces so as to avoid major accidents which can involve the destruction of a part or the whole of the exchanger.
It is therefore extremely important to detect possible escapes at the steam generator in which the heat exchanges between the liquid sodium and the water are carried out, for evaporation of the latter. The steam generator is generally constituted by tubes of small diameter inside which the water runs in order to be evaporated, the tubes being disposed inside the steam generator vessel in which liquid sodium flows which comes into contact with their outer surface. Although numerous precautions are taken during the manufacture and testing of the tubes used in steam generators, these tubes can nevertheless, after a longer or shorter working period of the power station, exhibit small fissures giving rise to escapes which can widen during subsequent working of the generator so as to result in the phenomenon of extremely rapid self-developing escape which can lead to the destruction of a considerable part of the steam generator.
It is therefore important to detect possible escapes while they are still very small and far removed from the self-developing state.
An escape in a steam generator tube, taking into account the high pressure at the water or water vapor side with respect to that of liquid sodium, is evidenced by a contamination of this liquid sodium by water or water vapor.
Various methods have been proposed for the very sensitive detection of traces of water in the liquid sodium in steam generators, and in particular measurement of the concentration of hydrogen in the liquid sodium at various points in the sodium circuit containing the steam generator is in current use.
To measure this concentration of hydrogen, liquid sodium sampled at the point in the steam generator at which measurement of the hydrogen concentration is to be made is put in contact with one of the faces of a wall of nickel, the other face of the nickel wall being subjected to a very low pressure. The flow of the hydrogen diffusing through the nickel wall into the medium under very low pressure is then measured, and from this the concentration of hydrogen in the liquid sodium sampled from the steam generator is deduced.
The steam generators of fast neutron nuclear power stations generally have systems of fixing the tubes containing water or steam which partition the sodium circuit into various sectors in which flows are practically independent of each other over the greater part of the height of the steam generator.
Inside each of these sectors, the liquid sodium which flows vertically presents, in the case of escape in one of the sectors, a concentration of hydrogen which is far greater than the hydrogen concentration in the sodium in the other parts of the steam generator.
To date, there has been no apparatus which allowed measurement of the concentration of a substance such as hydrogen in samples of liquid sodium originating from various points in an installation in which this liquid sodium circulates to be carried out rapidly and successively.
Selection of the measurement in chambers under vacuum of hydrogen separation apparatuses such as described hereinbefore, associated with each of the sampling points, allows this operation to be carried out in principle.
The problem posed is therefore in fact that of measuring a parameter relating to a substance whose presence has to be determined, in a certain number of chambers under reduced pressure, these measurements having to be made successively, rapidly and reliably, in each of the chambers. No apparatus capable of carrying out this operation in good conditions has been known hitherto.